The present invention relates to x-ray imaging apparatus and, more particularly, to a novel electrostatic x-ray imaging apparatus having a control mesh for improving imaging contrast.
Apparatus for recording an x-ray image by electrostatic techniques, particularly in a device utilizing air-exposable recording film capable of rapid processing by xerographic techniques, is highly desirable to physicians and other x-ray technology users. Apparatus is known which fullfills these requirements, as disclosed in U.S. Pat. No. 3,940,620, issued Feb. 24, 1976, and in U.S. Pat. No. 4,039,830, issued Aug. 2, 1977, both assigned to the assignee of the present invention. U.S. Pat. No. 3,940,620 discloses an electrostatic x-ray image recording device having a gas-filled gap enclosed by a pair of spaced electrodes. A first electrode has a layer of fluorescent material, emitting ultraviolet photons responsive to receipt of x-ray quanta, and an overlayer of an air-exposable ultraviolet-sensitive photoemitting material. The remaining electrode has a plastic sheet positioned adjacent thereto and within the gas-filled gap. The photoemitting material emits electrons, responsive to incident ultraviolet photons from the fluorescent material layer, and the photons are accelerated across the gap responsive to an electric field therein. The accelerated electrons are amplified in the gas-filled gap by an avalanche effect and are deposited upon the the plastic sheet to form an electrostatic image, subsequently developed by xerographic techniques.
U.S. Pat. No. 4,039,830 discloses an electrostatic x-ray imaging recording device having a pair of spaced electrodes with a gas-filled gap therebetween and having a layer of a fluorscent phosphor material, emitting ultraviolet photons responsive to x-ray excitation, formed upon the gap-facing surface of one electrode and having a plastic sheet adjacent the gap-facing surface of the other electrode. A conductive mesh is positioned within the gas-filled gap and supports photocathodic material upon the mesh surface facing the phosphor layer on the first electrode. An x-ray quanta impinging directly upon the photocathodic material generate "fast" electrons which are generally directed away from the plastic sheet while "slow" electrons, generated in the photocathodic material responsive to ultraviolet photons produced in the phosphor layer, are emitted and caused to pass through the interstices of the mesh, by an electric field generated across the inter-electrode gap, and are subjected to acceleration and amplification within the gap prior to deposition upon the plastic sheet, for later development by xerographic techniques.
In the apparatus of either of the aforementioned Letters Patent, the avalanche amplification gain in the gas-filled gap depends upon the nature of the gas filling the gap and upon the amplitude of the field across the gap and hence the electric field in the gas-filled gap. The accumulation of the charges on the charge-receiving dielectric layer will decrease the electric field in the gas-filled gap. A decreasing field in the gap produces decreasing avalanche amplification and results in deposition of decreasing amounts of charge in the charge image, whereby relatively poor image contrast results. It is desirable to provide improved apparatus of the aforementioned types which assure that substantially constant avalanche amplification occurs during the entire image formation interval whereby the contrast of the charge image deposited upon the plastic sheet is improved.